Static Data Members

[TimTrudeau]

I am getting the following error from g++ ...

./include/WatchTable_class.h:23: error: invalid use of non-static data member `WatchTable::start'

Here is the class header file ...

#ifndef WatchTable_class_h
#define WatchTable_class_h

#include "./watch_class.h"

class WatchTable
{
private:
Watch* start; //<<<<<<<<<<<<<<<<
int tableLength;

public:
WatchTable();
~WatchTable();
...
};

The indicated line is where the problem occurs. I am trying to create a pointer to an object of class Watch named start. I can find nothing on the web about what causes this error, but if I change the line to read...

static Watch* start; //<<<<<<<<<<<<<<<<

the error goes away %-) but then I get a link error...

undefined reference to `WatchTable::start'

I have no idea why the compiler wants start to be a static pointer, or why the linker can not find WatchTable::start after I changed it to static.

Thanks in advance for any help. I have been pouring over my books and the web but I can't get my head around this one.

[TimTrudeau]

Static Data Members
--------------------------------------------------------------------------------

I am getting the following error from g++ ...

./include/WatchTable_class.h:23: error: invalid use of non-static data member `WatchTable::start'

Here is the class header file ...

#ifndef WatchTable_class_h
#define WatchTable_class_h
 
#include "./watch_class.h"
 
class WatchTable
{
private:
Watch* start; //<<<<<<<<<<<<<<<<
int tableLength;
 
public:
WatchTable();
~WatchTable();
...
};
 

The indicated line is where the problem occurs. I am trying to create a pointer to an object of class Watch named start. I can find nothing on the web about what causes this error, but if I change the line to read...

static Watch* start; //<<<<<<<<<<<<<<<<
 

the error goes away %-) but then I get a link error...

undefined reference to `WatchTable::start'

I have no idea why the compiler wants start to be a static pointer, or why the linker can not find WatchTable::start after I changed it to static.

Thanks in advance for any help. I have been pouring over my books and the web but I can't get my head around this one.

[r035198x]

Please do not double post.

Threads merged.

[gpraghuram]

Hi,
If you are using any static variable inside a class then the variable needs to be initialized outside the class, otherwise you will get a linker error.

Init like this
Watch* WatchTable::start=NULL;

will solve the linker error.

If u can post a snippet of the the header file then it will be helpful to find the issue.
Thanks
Raghuram

[Banfa]

I don't think making start static will solve your problem as you then only get 1 instance of the member for all instances of the class. The compiler does not 'want' start to be static, you have just randomly added the keyword without understanding and fixing the problem.

The line of code you indicate looks good, I assume that Watch is defined in watch_class.h, presumably as a class from the name of the header file. Which suggests the error is else where in your code.

Did you get any other warnings or errors that you have not reported?

[jesusdiehard]

Making the Watch member variable static is the not the solution.

Compiler is not getting the definition of Watch which i supposed should be in Watch_Class header file. Can u print the error u get which u get when u make the Watch member variable non-static or print the snippet of header file Watch_Class.

[TimTrudeau]

Thanks for the help to all. I found the problem. It is correct that I did not want start to be static. The compiler error message had me confused. The problem was caused because I tried use the class member 'start' as a default parameter value in one of the member functions:

Watch* find(EUI64& _eiu, Watch* _w=start);
 

I changed the line above to:

Watch* find(EUI64& _eiu, Watch* _w=NULL);
 

and now all is good!